FIG. 1 and FIG. 2 are a hydraulic circuit diagram and a cross sectional view of a flow control valve for construction equipment having a floating function of the related art.
As illustrated in FIG. 1 and FIG. 2, the flow control valve includes: a valve body 7 defining a supply passage communicating with a pump passage 2 through which working fluid is supplied from a hydraulic pump 1 and first and second actuator passages 5 and 6 connected to a hydraulic cylinder 4 actuated by working fluid supplied from the hydraulic pump 1;
a spool 9 switchably disposed within the valve body 7, wherein the spool 9 is switched to allow the supply passage 3 to communicate with the first or second actuator passage 5 or 6, such that an amount of working fluid from the hydraulic pump 1 is supplied to the hydraulic cylinder 4 through the supply passage 3 and the first actuator passage 5, and an amount of working fluid discharged from the hydraulic cylinder 4 returns to a tank passage 8 through the second actuator passage 6;
a regeneration passage 10 through which a portion of working fluid returning to the tank passage 8 from a large chamber of the hydraulic cylinder 4 is supplied to a small chamber of the hydraulic cylinder 4 such that the portion of working fluid is regenerated;
a floating switching valve 11 including a logic valve 11a configured to open and close a passage 5a branched from the first actuator passage 5, wherein the floating switching valve 11 is switched in response to a pilot pressure c applied thereto to drain working fluid from a back pressure chamber of the logic valve 11a through a control valve 11b and a drain line dr2, thereby opening the passage 5a, such that, when a floating function of causing the large chamber and the small chamber of the hydraulic cylinder 4 to communicate with each other is selected, the large chamber and the small chamber of the hydraulic cylinder 4 communicate with each other, and a portion of working fluid from the large chamber and the small chamber communicating with each other is connected to a working fluid tank T; and
a logic valve 12 openably and closably disposed in the first actuator passage 5 to prevent a boom from moving downwardly due to contraction of the hydraulic cylinder 4 caused by an oil leakage.
A) A case of lifting the boom by actuating the hydraulic cylinder 4 will be described.
When the spool 9 is switched to the right on the drawing in response to a pilot pressure a being applied thereto, an amount of working fluid from the hydraulic pump 1 is supplied to the large chamber of the hydraulic cylinder 4 through sequentially, the pump passage 2, the supply passage 3, the spool 9, the first actuator passage 5, and the logic valve 12. At this time, an amount of working fluid discharged from the small chamber of the hydraulic cylinder 4 returns to the working fluid tank T through sequentially, the second working fluid passage 6, the spool 9, and the tank passage 8.
Thus, the stretching of the hydraulic cylinder 4 (a so called boom cylinder) can lift the boom (boom up).
B) A case of lowering the boom by actuating the hydraulic cylinder 4 will be described.
When the spool 9 is switched to the left on the drawing in response to a pilot pressure b applied thereto, an amount of working fluid from the hydraulic pump 1 is supplied to the small chamber of the hydraulic cylinder 4 through sequentially, the pump passage 2, the supply passage 3, the spool 9, and the second actuator passage 6.
At this time, a pilot pressure b1 is applied to the control valve 15, such that an amount of working fluid from the back pressure chamber 12a of the logic valve 12 communicates with the first actuator passage 5 through the control valve 15 to open the logic valve 12. Then, an amount of working fluid discharged from the large chamber of the hydraulic cylinder 4 returns to the working fluid tank T through sequentially, the logic valve 12, the first actuator passage 5, the spool 9, the regeneration passage 10, a booster valve 13, and the tank passage 8.
When the pressure of working fluid within the regeneration passage 10 is higher than the pressure within the second actuator passage 6, a portion of working fluid in the regeneration passage 10 may merge with working fluid in the second actuator passage 6 through a check valve 14 disposed in the regeneration passage 10, thereby being supplied to the small chamber of the hydraulic cylinder 4.
Consequently, the contraction of the hydraulic cylinder 4 can lower the boom (boom down).
C) A case of performing a floating function will be described.
When a pilot pressure c is applied to the control valve 11b of the floating switching valve 11, the control valve 11b is switched to the left on the drawing, an amount of working fluid drains from the back pressure chamber of the logic valve 11a through the control valve 11b and the drain line dr2. That is, when the control valve 11b is switched, the large chamber and the small chamber of the hydraulic cylinder 4 communicate with each other, and a portion of working fluid within the communicating large and small chambers flows to the working fluid tank T.
Since separately from a main control valve (MCV) is provided a floating switching valve 11, which provides a floating function allowing a bucket B to move along an irregular surface E to perform ground leveling work, as illustrated in FIG. 7, the number of parts increases, thereby increasing the manufacturing cost. In addition, since the floating switching valve 11 is additionally provided, the layout of equipment becomes complicated, and cost for the floating switching valve 11 is additionally caused, which are problematic.